We're also going to talk a lot about light in this class.
And so, you need to be familiar with the various
ways of describing the different kinds of light that are found.
A light ray consists of waves that
we'll talk more about in future lectures.
But one way to characterize the different kinds of
light waves is by the wavelength,
the distance between two adjacent peaks.
It's given the Greek letter lambda to describe the wavelength.
And the units of wavelength are the number of cycles per centimeter.
I have this upside down here.
No, centimeter, centimeters per cycle, that's centimeters per cycle.
And a lot of times, the units of cycle are left
out, sort of implicit.
And so sometimes very often you will
see wavelength described in units of centimeters.
Another way to describe light is in terms of the frequency of light.
So light is moving at some speed C, the speed of light.
And so, if you're just sitting here watching this light
go past you, you can count the peaks as they're coming.
And so, the frequency of the light, which is given
the Greek letter nu, has units of cycles per second.
Or, since they tend to drop the cycles, you can see this written as just
one over seconds, or seconds to the minus 1.
Can be confusing if you don't know what's going on.
But it's not that hard.
So if we want to get some more factor label
practice at converting units, this is a good example.
Let's say we have a wavelength of light and we want to calculate its frequency.
So the frequency has units of cycles per second.
So that's what we want.
And looking at the wavelength,
the wavelength itself is centimeters per cycle,
but it looks like we want the cycles to be on top.
So I'm going to put the wavelength upside-down, one over lambda.
And so that gives us units here of
cycles per centimeter, the inverse of lambda there.
And then the speed of light C can be used to cancel the centimeters and get us
the cycles per second that we're looking for.
Another metric of light that turns out to be useful
when we start talking about infrared light is called the wave number.
And the wave number is simply the amount,
the number of waves within 1 centimeter of distance.
So you count the number of peaks in there,
and the units of that are cycles per centimeter.
Or by neglecting, ignoring the cycles there, it's oftentimes written as
1 / cm, or cm raised to the power of minus 1.
The wave number is just given the regular letter n.